Electromechanical pressure transducer

ABSTRACT

An improved ceramic capacitive pressure sensor adaptable for use in automobiles is disclosed. The pressure sensor comprises a thin flexible diaphragm disc having a capacitor electrode deposited on it. A thick cylindrical base substrate has a top surface with a second capacitor electrode deposited thereon and the diaphragm is bonded to the top surface of the base substrate by an annular glass ring such that the first and second electrodes are separated from each other and face each other directly across an air gap. The diaphragm, the top surface of the base substrate and the annular glass ring form an internal cavity in which a reference vacuum is stored. In response to pressure changes outside of this cavity, the diaphragm will flex and thereby change the capacitance created by the first and second electrodes. At least one channel portion is provided in the base substrate top surface which opens into the cavity storing the reference vacuum. This improves the electrical and mechanical characteristics of the pressure sensor. The diaphragm electrode is substantially larger than the base electrode and extends substantially over all of the central portion of the diaphragm within the glass annular ring and in this manner the electrode acts as a shield against conductive elements which are exterior to the internal cavity and are adjacent to the thin diaphragm. Also, a large electrode is provided on the diaphragm opposite to the diaphragm electrode, this large electrode being coupled to a metallic housing through a conductive gasket to obtain a similar shielding effect.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field ofelectromechanical pressure sensors and more particularly to the field ofceramic capacitive pressure transducers. Ceramic capacitive pressuretransducers are known and generally comprise parallel plate capacitorelectrodes separated by an air gap wherein the spacing between theparallel plate electrodes is altered in response to a sensed pressurethereby changing the capacitance created by these electrodes. Generally,one capacitor electrode plate is deposited on a top end surface of arelatively thick cylindrically shaped ceramic base substrate while theother capacitor electrode is deposited on a relatively thin disc-shapedceramic pressure sensing diaphragm. An annular glass insulating ring isdeposited on the peripheral portion of the base substrate top surfaceand is used to bond the diaphragm to the base substrate as well as tospace the diaphragm electrode a predetermined distance away from thebase substrate electrode. Typically, the diaphragm, the annular glassring and the base substrate are assembled into a sandwich type structureand then heated to form an integral assembly such that the capacitorelectrodes are spaced apart by a predetermined distance totallydependent upon the thickness of the annular glass ring. Generally, avacuum entryway hole is provided through the base substrate, and throughthis hole a predetermined reference vacuum pressure is applied to an aircavity formed by the diaphragm, the annular glass ring and the basesubstrate. Subsequently, the vacuum entryway is sealed so that theinternal cavity will maintain (store) a predetermined reference vacuumpressure. By applying various degrees of pressure to the exterior of thecapacitive pressure transducer, the transducer diaphragm is flexed bypredetermined amount and this results in changing the capacitancecreated by the capacitor electrodes since the flexing of the diaphragmchanges the spacing between the electrodes. Thus by monitoring thecapacitance created by the electrodes, the ceramic capacitor transducerwill produce an electrical signal related to the magnitude of theexterior pressure applied to the diaphragm as compared to the magnitudeof the reference vacuum pressure. Such transducers are readily adaptablefor sensing vacuum pressures generated by automobile internal combustionengines.

Typically, the nominal distance between the base and diaphragm capacitorelectrodes is very small so that small changes in exterior pressure willresult in relatively large changes in the capacitance created by theseelectrodes. In prior art ceramic capacitor transducers of this type, thetop surface of the base substrate, as well as the diaphragm surface onwhich the diaphragm electrode is deposited, are substantially planar.Because of this and because the nominal distance between the electrodeshas to be kept relatively small, the volume of the internal cavity whichstores the reference vacuum pressure is very small. This results inthese prior art capacitive transducers having relatively short lifetimes whenever any appreciable leakage rates for the internal cavityexist. Also, since previous ceramic capacitive transducers have basesubstrates which are substantially solid, except for a narrow vacuumentryway hole, the base capacitor electrodes is surrounded with ceramicmaterial having a high dielectric constant, and the end result is thatthe capacitor is much more susceptible to capacitive fringing effects.In addition, prior art capacitive transducers encounter capacitivefringing problems due to the effect of conductive surfaces or particleswhich are located exterior to the internal cavity but close to the thinflexible diaphragm. These conductive surfaces or particles createvariable additional coupling between the capacitor electrodes andtherefore tend to make these capacitive pressure sensors unrealiable inthat a fixed value of capacitance would not always be generated inresponse to a fixed value of external pressure being applied to thediaphragm.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improvedelectromechanical pressure sensor which overcomes the aforementioneddeficiencies.

A more specific object of the present invention is to provide animproved capacitive pressure transducer which is less susceptible tofringing effects caused by conductive surfaces and particles exterior tothe pressure transducer.

According to the present invention an improved pressure transducerhaving a pressure displaceable diaphragm is provided in which theimproved transducer is less susceptible to the presence of conductiveelements adjacent to a pressure sensing displaceable diaphragm. Thetransducer comprises; base substrate means having top and bottomsurfaces; flexible diaphragm means having a planar central portion and aperipheral portion; and mounting means for mounting said peripheralportion of said diaphragm means to said base substrate top surface, saidmounting means, said diaphragm planar central portion and said substratetop surface forming an internal cavity, said diaphragm central portionbeing displaceable with respect to said top surface in response topressure changes to thereby alter the electrical characteristics of thetransducer and wherein the diaphragm has a guard electrode meansextending over substantially all of said central portion of saiddiaphragm for shielding the transducer against conductive elementsexterior to the internal cavity and located adjacent to the flexiblediaphragm. Basically the present invention also contemplates providing aguard electrode over substantially all of an inner planar portion of acapacitive transducer diaphragm. The ground electrode means issubstantially larger than a capacitance electrode on the base substrateand this minimizes the variable effects of conductive elements exteriorto the pressure transducer assembly. Also, a large electrode is providedon the diaphragm opposite to the diaphragm electrode, this largeelectrode being coupled to a metallic housing through a conductivegasket to obtain a similar shielding effect.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention reference should madeto the drawings, in which:

FIG. 1 is a perspective view of a metallized base substrate of a ceramiccapacitive pressure transducer;

FIG. 2 is a plane view of a metallized ceramic diaphragm of a ceramicpressure transducer; and

FIG. 3 is a cross-sectional view of a ceramic capacitive pressuretransducer mounted in a housing and utilizing the components shown inFIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a relatively thick cylindrically shaped ceramicbase substrate 10 of a ceramic capacitive pressure transducer isillustrated. The base substrate 10 has a thickness T, a substantiallycircular top end surface 11, a substantially planar circular bottom endsurface 12 and through holes 13 and 14 located along a planar annularperipheral portion of the circular top surface 11 and extending from thesurface 11 to the surface 12. An annular ring 15 of insulating glassmaterial is provided on the peripheral portion of the top surface 11.Within this glass annular ring, a vacuum entryway through hole 16 isprovided and extends from the top surface 11 to the bottom surface 12. Ametallic electrode area 17 is provided on the top surface 11 andcomprises a substantially circular metallization portion 18 locatedwithin the annular ring 15 and a radially extending finger portion 19extending from the circular electrode portion 18, underneath the glassinsulating ring 15 to the through hole 14. A generally C-shaped channel20 having a depth D (shown in FIG. 3) opens onto the top surface 11 andis positioned within said annular glass ring 15 such that it surroundsthe circular electrode portion 18 and such that the radially extendingfinger 19 passes through the open portion 21 of the C-shaped channel 20.The electrode portions 18 and 19 are deposited on coplanar portions ofthe surface 11 which are also coplanar with the planar annularperipheral portion of the surface 11.

FIG. 2 illustrates a relatively thin ceramic disc 22 which functions asa diaphragm means for a ceramic capacitor transducer. The disc 22 has aplanar circular surface 23 upon which a circular metallization area 24is deposited, the diameter of the circular metallization 24 beingsubstantially greater than the diameter of the circular metallization 18on the base substrate 10. The metallization 24 extends oversubstantially all of a central portion of the surface 23 and a radialfinger projection of metallization 25 extends from the circular area 24radially outward into an annular peripheral portion of the surface 23which surrounds the metallization 24.

FIG. 3 illustrates an improved pressure transducer assembly whichcomprises the base substrate 10 illustrated in FIG. 1 and the diaphragmdisc 22 illustrated in FIG. 2, assembled together and mounted in agenerally rectangular in cross section transducer housing. Identicalreference numbers are used to identify components in FIG. 3 whichcorrespond to the components shown in FIGS. 1 and 2.

Basically, the diaphragm disc 22 is mounted to the base substrate 10 bythe annular glass ring 15 bonding the peripheral portion of the surface23 surrounding the metallization 24 to the annular peripheral portion ofthe base substrate surface 11. The annular glass ring 15, the planarcentral portion of the diaphragm surface 23 within the annular glassring 15 and the top surface 11 of the base substrate 10 within theannular glass ring 15 form an internal cavity 31, and the vacuumentryway 16 extends from the bottom surface 12 of the base substratethrough the base substrate into the cavity 31. A sealing structure 32 ispresent on the surface 12 and effectively seals the vacuum entryway 16which opens onto the surface 12 such that the cavity 31 is essentiallyair tight. This occurs since the diaphragm disc 22 was bonded to thebase substrate 10 by assembling the components as shown in FIG. 3 andthen subjecting the assembly to a high temperature at which the glassannular ring 15 softened and thereby bonded the disc 22 to the substrate10 with a substantially hermetic seal.

A reference vacuum pressure is stored in the cavity 31 by applying avacuum to this cavity through the vacuum entryway 16 and subsequentlysealing this entryway by the sealing means 32. In this manner, thecavity 31 is maintained at a vacuum reference pressure which results inflexing the diaphragm disc 22 such that a predetermined spacing existsbetween the electrode metallizations 18 and 24. When various pressuresare applied exterior to the cavity 31 of the pressure transducercomprising the diaphram 22 and the base substrate 10, these variouspressures will cause the disc 22 to flex and thereby slightly increaseor decrease the spacing between the electrode metallizations 18 and 24.This in turn results in changing the capacitance represented by thesetwo electrode metallizations and it is this capacitance change which ismonitored by not shown electrical circuitry to produce an indication ofthe magnitude of the external pressure being applied to the diaphragmdisc 22.

FIG. 3 illustrates that a first external electrical connection lead 33is inserted through the peripheral through hole 14. It is contemplatedthat this external lead will be connected by some suitable conductivemeans to the finger projection metallization 19. Thus lead 33 representsthe external lead connection to the metallization 18 which serves as oneelectrode of a parallel plate capacitor. Similarly, an external lead 34is contemplated as passing through the peripheral through hole 13 suchthat it will electrically contact the radial finger metallization 25 ofthe diaphragm disc 22. In this manner, the external lead 34 will beconnected to the metallization 24 that serves as the other electrode ofa parallel plate capacitor.

The transducer comprising the flexible diaphragm 22 and the basesubstrate 10 is contemplated as being mechanically mounted in somemanner within the transducer housing 30. In FIG. 3, one such mounting isillustrated in which the outer peripheral area of the disc 22 rests on acircular rubber O-ring 35 which is held in place by shoulders 36 formedon the interior wall of the housing 30. A through hole 37 is provided inthe housing 30 and the housing, the O-ring 35 and the diaphragm disc 22essentially formed an exterior cavity 38 to which various pressures areapplied via the opening 37. It is these various pressures which exist inthe cavity 38 that are sensed by the pressure transducer. Thus thecapacitance created between the external leads 33 and 34 represents ameasure of the various pressures applied to the cavity 38 with respectto the vacuum reference pressure stored in the internal cavity 31.

Preferably, the metallizations 18, 19, 24 and 25 are screen printed ontothe ceramic base substrate 10 and the ceramic disc 22, respectively. Inaddition, the annular glass ring 15 is also contemplated as being screenprinted onto the surface 11 of the base substrate 10. Preferably, theannular glass ring 15 will comprise a thick film glass paste whichcontains glass particles that will soften at a moderately hightemperature and thereby bond the peripheral portion of the disc 22 tothe annular peripheral portion of the surface 11 of the base substrate10. Any type of convenient sealing mechanism can be used for the sealingmeans 32 since the function of this means is just to close off thevacuum entryway 16 after an appropriate reference vacuum pressure hasbeen applied to the cavity 31.

Providing the base substrate 10 with the C-shaped channel 20 which opensonto the surface 11 essentially increases the total volume available forthe internal cavity 31. This means that the lifetime of the pressuresensor will be increased since the cavity 31 can now maintain areference vacuum pressure for a substantially longer period of timedespite any pressure leakage through the substrate 10, the annular glassring 15 or the ceramic diaphragm 22. In addition, providing the channel20 around the circular electrode area 18 will reduce the fringing fieldcreated at the edge of this electrode. This occurs because now theelectrode 18 is proximately surrounded by the gap created by the channel20 rather than being surrounded by a ceramic material which wouldtypically have a high dielectric constant. the ceramic materialpreferably used for the base 10 and disc 22 of the present invention isalumina (Al₂ O₃) which has a dielectric constant of 9.

The fact that the diaphragm electrode area 24 is substantially greaterthan the base substrate electrode area 18 produces a shielding effect inwhich conductive elements that are located exterior to the cavity 31 butstill somewhat adjacent to the thin diaphragm 22 will now have a minimumeffect on the capacitance created by the capacitor electrodes 18 and 24.If the capacitor electrodes were of substantially equal size, then asubstantial fringing field would exist at the edges of the capacitorelectrodes and conductive contaminants located adjacent to the diaphragm22 and exterior to the cavity 31 would have parasitic capacitivecoupling effects between the capacitor electrodes and thereby create avariable and unpredictable capacitance reading. For the typical housingof the pressure transducer illustrated in FIG. 3, it is likely thatwater droplets may be present in the cavity 38. The presence of thesedroplets would thereby severely effect the capacitance generated by theplates 24 and 18 unless the effect of these contaminants was shieldedagainst. The present invention provides this shielding by essentiallymasking the effect of conductive surfaces or particles located adjacentto the diaphragm 22 but exterior to the cavity 31.

A shielding effect similar to that produced by having the capacitorelectrode 24 substantially larger than the electrode 18 can be producedby having the electrodes 24 and 18 substantially the same size butcoating the planar circular surface of the disc 22 bordering on thecavity 38 with a conductive metallization 100, such as graphite or athick film metallization. This exterior conductive metallization couldthen be coupled to the housing 30, which would preferably be metallic,by making the rubber O-ring 35 also conductive and thereby function as aconductive gasket. In this manner the exterior electrode on the disc 22bordering the cavity 38 would now provide the shielding effect producedby the oversize electrode area 24 shown in FIG. 3. Using either of thesetwo techniques, an effective shielding can be provided such thatvariable conductive contaminants which may be present in the cavity 38will have a minimum effect on capacitance created by the electrodes ofthe pressure transducer.

While I have shown and described specific embodiments of this invention,further modifications and improvements will occur to those skilled inthe art. All such modifications which retain the basic underlyingprinciples disclosed and claimed here are within the scope of thisinvention.

I claim:
 1. A pressure transducer in which pressure changes displace adiaphragm whose position determines electrical characteristics of saidtransducer, comprising:base substrate means having top and bottomsubstantially planar end surfaces, said top end surface having asubstantially planar central portion; flexible diaphragm means,relatively thin with respect to the distance between said top and bottomend surfaces, having a planar central portion and a peripheral portion;mounting means for mounting said peripheral portion of said diaphragmmeans to said base substrate top surface, said mounting means, saiddiaphragm planar central portion and said substrate top surface planarportion generally forming an internal cavity, said diaphragm centralportion being displaceable with respect to said top surface in responseto pressure changes, and wherein said cavity maintains a fixed referencepressure and pressures applied exterior to said internal cavity causesaid diaphragm means to flex and thereby alter electricalcharacteristics of said transducer in response to pressure changes; saidpressure transducer being a capacitive transducer having a firstelectrode on said top surface central planar portion of said basesubstrate and a second electrode on said diaphragm means planar centralportion, said first and second electrodes directly facing each otheracross and separated from each other by a gap forming a portion of saidinternal cavity; a metallic housing for said transducer; and whereinsaid diaphragm means has conductive guard electrode means extending oversubstantially all of said planar central portion of said diaphragm meansfor shielding against conductive elements exterior to said internalcavity and adjacent to said diaphragm means which may create variableeffects in the electrical characteristics of said transducer; saidtransducer including connecting means comprising a flexible conductivegasket located on a surface of said diaphragm means parallel to saidplanar central portion of said diaphragm means that forms a boundary forsaid internal cavity, and wherein said guard electrode means comprises aconductive metallization area substantially larger than said firstelectrode area and deposited on the surface of said diaphragm means incontact with said flexible conductive gasket, said conductive gasketelectrically connecting said metallic housing and said guard electrodemeans.
 2. A pressure transducer according to claim 1 wherein saiddiaphragm means is a disc-shaped ceramic plate and said base substratemeans comprises a cylindrical shaped ceramic substrate.
 3. A pressuresensor according to claim 2 wherein said mounting means comprises aannular glass ring bonding said diaphragm means to said base substratetop surface.